Theoretical calculation of equilibrium Mg isotope fractionation between silicate melt and its vapor

Isotope fractionation during the evaporation of silicate melt and condensation of vapor has been widely used to explain various isotope signals observed in lunar soils, cosmic spherules, calcium-aluminum-rich inclu- sions, and bulk compositions of planetary materials. During evaporation and condensation, the equilibrium isotope fractionation factor （α） between high-temperature silicate melt and vapor is a fundamental parameter that can con- strain the melt＇s isotopic compositions. However, equilib- rium a is difficult to calibrate experimentally. Here we used Mg as an example and calculated equilibrium Mg isotope fractionation in MgSiO3 and Mg2SiO4 melt-vapor systems based on first-principles molecular dynamics and the high- temperature approximation of the Bigeleisen-Mayer equation. We found that, at 2500 K, 625Mg values in the MgSiO3 and Mg2SiO4 melts were 0.141 ±0.004 and 0.143 ±0.003‰ more positive than in their respective vapors. The corresponding 626Mg values were 0.270 ± 0.008 and 0.274 ± 0.006‰ more positive than in vapors, respectively. The general α - T equations describing the equilibrium Mg α in MgSiO3 and Mg2SiO4 melt-vapor systems were： αMg（l）-Mg（g） = 1 ＋ 5.264×10^5/T^2 （1/m - 1/m＇） and αmg（l）-Mg（g） = 1 ＋ 5.340×10^5/T^2 （1/m - 1/m＇）, respectively, Where m is the mass of light isotope, ^25Mg or ^26Mg. These results offer a necessary parameter for mechanistic under- standing of Mg isotope fractionation during evaporation and condensation that commonly occurs during the early stages of planetary formation and evolution.

Equilibrium and kinetic Si isotope fractionation factors and their implications for Si isotope distributions in the Earth's surface environments

Several important equilibrium Si isotope fractionation factors among minerals,organic molecules and the H_4SiO_4 solution are complemented to facilitate the explanation of the distributions of Si isotopes in Earth's surface environments.The results reveal that,in comparison to aqueous H_4SiO_4,heavy Si isotopes will be significantly enriched in secondary silicate minerals.On the contrary,quadra-coordinated organosilicon complexes are enriched in light silicon isotope relative to the solution.The extent of ^(28)Si-enrichment in hyper-coordinated organosilicon complexes was found to be the largest.In addition,the large kinetic isotope effect associated with the polymerization of monosilicic acid and dimer was calculated,and the results support the previous statement that highly ^(28)Sienrichment in the formation of amorphous quartz precursor contributes to the discrepancy between theoretical calculations and field observations.With the equilibrium Si isotope fractionation factors provided here,Si isotope distributions in many of Earth's surface systems can be explained.For example,the change of bulk soil δ^(30)Si can be predicted as a concave pattern with respect to the weathering degree,with the minimum value where allophane completely dissolves and the total amount of sesquioxides and poorly crystalline minerals reaches their maximum.When,under equilibrium conditions,the well-crystallized clays start to precipitate from the pore solutions,the bulk soil δ^(30)Si will increase again and reach a constant value.Similarly,the precipitation of crystalline smectite and the dissolution of poorly crystalline kaolinite may explain the δ^(30)Si variations in the ground water profile.The equilibrium Si isotope fractionations among the quadracoordinated organosilicon complexes and the H_4SiO_4solution may also shed light on the Si isotope distributions in the Si-accumulating plants.

First-principle study of Ba isotopic fractionation during ion exchange processes

The potential utilization and development of the Ba isotope tool depend on an accurateδ^(137/134)Ba determination of the samples.During the chemical purification,whether the adsorption process on the surface of the ionexchange resin could lead to the Ba isotopic fractionation and the degree of fractionation directly influence the accurateδ^(137/134)Ba determination.In the present work,first-principles calculations based on the density functional theory were used to quantify the Ba isotopic equilibrium fractionation factor between the aqueous solution and the resin in the acid leaching process.By constructing and optimizing the geometric configurations of Ba-containing species,Ba(H_(2)O)_(n)^(2+),Ba(H_(2)O)_(n)Cl_(2),Ba(H_(2)O)_(n)(NO_(3))2,and the adsorbed Ba^(2+)on the surface of the resin,extracting the harmonic vibrational frequencies,we finally at 298 K obtained the fractionations,Δ^(137/134)Ba_(soln-ads)=0.07‰,Δ^(137/134)Ba_(Ba(H_(2)O)_(n)Cl_(2)-ads)=0.05‰,andΔ^(137/134)-Ba^(Ba(H_(2)O)_(n)(NO_(3))2-ads)=0.02‰.Overall,there were almost no Ba isotope fractionations during leaching.Although the Ba isotope fractionation can be magnified by the Rayleigh fractionation process in purification,the difference inδ137/134Ba between the initial and final stages did not exceed0.060‰(or 0.045‰)when leaching the standard sample with HCl or HNO_(3),which is equal to or less than the accuracy of Ba isotopic analysis.At a common yield of89.75%,Ba isotopic fractionation induced by incomplete recovery was 0.015‰for HCl(or 0.011‰for HNO_(3)).Finally,if the influence of an incomplete recovery on theδ137/134Ba determination needs to be ignored,the recovery is suggested to be not less than 67%for HCl(or 46%for HNO_(3)).

Boron isotope geochemistry of Zigetang Co saline lake sediments,Tibetan Plateau

The origin of boron in boron-rich salt lakes in the Tibetan Plateau is highly controversial.In this study,we carried out a detailed study on boron geochemistry and isotope composition of lake sediments collected in Zigetang Co,central Tibet.Evaporites had high boron concentrations of 172.3–418.6 lg/g and δ^(11)B values of-8.2%to-3.3%,suggesting a non-marine origin for the saline lake.The boron isotopic fractionation factor,a,between evaporite and brackish water(a_(evaporite–brackish))decreased systematically with depth,from 0.9942 at the top of the drill core to 0.9893 at the bottom;the linear variation between α_(evaporite–brackish)and depth reflects boron isotopic fractionation associated with progressive crystallization.The positive correlation between δ^(11)B versus[B]and δ^(11)B versus depth in the evaporite phase reflects pH and boron speciation in the solution control on the adsorption of boron,and B(OH)_3 species incorporated preferentially into Mg(OH)_2 precipitation at high pH.